There is known a method of chromatographic analysis of mixtures of liquid substances (cf., e.g., U.S. Pat. No. 3,798,973) in which a sample of substances to be analyzed is introduced by a syringe to the evaporator of a gas chromatograph by piercing a septum of a self-sealing material and the sample in injected to the interior of an evaporation chamber pneumatically communicating with the inlet of a chromatographic column. Vapours of the sample generated in the interior of the evaporation chamber are entrained by the flow of a carrier gas to the chromatographic column where the mixture of substances under analysis is chromatographically separated into separate constituents. The separated constituents of the sample are detected at the outlet from the chromatographic column by a suitable detector (heat conductivity detector, plasma-ionization detector, etc.) to register the signal of the detector by a recorder and use the chromatogram for calculating the concentration of the substances under analysis.
In most of the prior art gas chromatographs using the aforedescribed method of analysis precision and reproducibility of the results of the analysis are influenced by the septum of a self-sealing material (such as silicone rubber) provided in the evaporator of the gas chromatograph in the passage for receiving the syringe needle. Such a septum usually leads to ghost peaks appearing in the chromatogram during programmable heating of the column even when a sample is not injected into the evaporator. When introducing the sample by a syringe the needle of which pierces the self-sealing septum, particles of the septum material are cut off the septum and are carried by the needle to the interior of the evaporation chamber or to the inlet of the chromatographic column. These particles can be the source of gas emission distorting the results of chromatographic analysis. In addition, the self-sealing septum suffers from a rapid loss of hermeticity and needs frequent replacement.
There is also known a method of chromatographic analysis of mixtures of liquid substances using a device for introducing the sample to a chromatographic column without a septum (cf., e.g., U.S. Pat. No, 4,414,857). According to this method, a sample of liquid mixture is introduced by a syringe or some other sample carrier to the evaporator of a gas chromatograph through an open passage or channel sealing this channel by a sealing element connected to the sample carrier in the course of sample injection, evaporation and transfer of the vapours of the sample material from the evaporator to the chromatographic column. The vapours of the sample material are transferred from the evaporator to the chromatographic column by a flow of carrier gas, which forces the vapours through a flow restrictor of constant cross-section arranged between the evaporation chamber and the inlet to the chromatographic column. After transferring the vapours of the sample material from the evaporator to the chromatographic column admission of the carrier gas to the evaporator is terminated and the flow of this gas is directed to the inlet of the chromatographic column after the outlet from the flow restrictor of constant cross-section. A minor part of the flow of carrier gas entering the evaporation chamber through the flow restrictor acts to clean the interior of the evaporator from the remainder of the sample material venting this remainder to the atmosphere.
The heretofore described method of chromatographic analysis is very efficient and makes it possible to obviate many disadvantages of the prior art methods employing self-sealing septums. However, it ensures analysis only when a preliminarily volatilized sample is fed to the column. This in turn limits the range of liquid substances to be analyzed making it impossible to analyze certain substances decomposable under the action of high temperatures in the evaporator of the gas chromatograph.
There is known one more method of chromatographic analysis, in which a liquid sample is introduced by a syringe having a special needle directly to the inlet of a chromatographic column (on-column injection) at a temperature below the boiling point of the substances being analyzed. The syringe needle is inserted to the column through a passage for receiving a sample carrier closed by a rotary valve. Before injecting the sample the passage for receiving the sample is opened by turning the valve to a respective position. Subsequent to introducing the sample the passage for receiving the sample carrier is closed and the temperature in the column is gradually raised, while transferring the substances being analyzed in a vaporous state by the carrier gas to the main separating section of the column, where the sample material is separated into constituents to be detected at the outlet from the column by a suitable detector (cf., e.g., U.S. Pat. No. 4,269,608).
The gas chromatograph for carrying out this method comprises a capillary chromatographic column secured inside a constant-temperature chamber with a programmable temperature control, a passage for introducing the sample pneumatically communicating with the capillary chromatographic column, a means for sealing and unsealing the passage for receiving a sample carrier in the form of a rotary valve provided in this passage, a pipe for feeding a carrier gas communicating with the passage for introducing the sample, and a detector provided at the outlet of the chromatographic column.
A liquid sample is injected to the inlet of the column in the absence of hermetic sealing of the passage for introducing the sample, i.e., without a flow of carrier gas from the inlet to the outlet of the chromatographic column. This results in uncontrollable losses of the substance under analysis due to the escape of vapours of the solvent through the passage for receiving the sample carrier to the atmosphere and in a tendency of some of the liquid sample to stick to the outer surface of the syringe needle. This latter effect is generally caused by drawing of a quantity of the sample to an annular space between the outer surface of the syringe needle and the walls of the capillary column by virtue of capillary forces (cf., e.g., K.Grob, Jr., Journal of Chromatography, 283, 1984, pp. 21 to 25), which affects the accuracy of chromatographic analysis.
In addition, the known method and device for carrying out this method fail to carry out a chromatographic analysis when splitting the sample at the inlet to the main section of the chromatographic column, which limits the range of their practical application. The gas chromatograph for effecting the method is structurally overcomplicated due to the provision of a rotary valve in the passage for receiving the sample carrier, which adversely affects a combination of the gas chromatograph of this type with automatic metering devices for liquid samples (automatic samplers). Attempts to expand the functional capabilities of the gas chromatograph of this type resulted in a greater structural overcomplication, and as a consequence in reduced reliability (c.f., EP, B, No. 0,140,020).